For a description of a wind turbine blade layout the following definitions are applied. Tip point (4) means the maximum radial position of the blade, measured from the blade root (2). Radius (5) means the distance measured from the turbine blade root (2), by which the blade is connected to a rotor hub, along the centreline which connects the blade root (2) with the tip point (4). Chord (6) means the maximum width of the turbine blade perpendicular to the centreline (16). Shoulder (3) means the radial position, measured from the blade root (2), where the chord distribution has its peak value. Maximum chord interval (7) means the radial interval over which the blade chord (6) is no less than 95% of the shoulder chord. Outer blade interval (8) means the radial interval located between the shoulder and the tip point over which the blade chord (6) is less than 95% of the shoulder chord. Root interval (17) means the radial interval located between the blade root (2) and the shoulder over which the blade chord (6) is less than 95% of the shoulder chord. For further illustration of the mentioned terms see also FIG. 1.
The layout of a wind turbine rotor blade is a result of a multidimensional and multi-functional process, where factors such as the annual energy production (AEP) of the wind turbine, the operational loads, the manufacturing process, the choice of materials, and secondary issues such as transport considerations are elements that all influence the final selection of optimum layout.
The common practice for wind turbine blade layout, in particular the chord distributions, is based mainly on rules of thumb or in some cases more complex optimisations based on maximising the annual energy production.
The document FR 575.990 discloses at page 2 in line 4 to 23 and in FIG. 2 an improved wind turbine blade, which is based on an originally rectangular shaped wind turbine blade. The improved wind turbine blade comprises a nearly rectangular shaped outer blade interval and an also nearly rectangular shaped maximum chord interval.
The paper “Structural design of a composite wind turbine blade using finite element analysis” by M. E. Bechly and P. D. Clausen, COMPUTER & STRUCTURES, vol. 63, no. 3, pages 639-646 describes preliminary work to optimise the use of material in a 2.5 m long fibreglass composite wind turbine blade. The radial variation of the blade chord and twist were determined using conventional blade element theory and the relative radial stacking arrangement of these blade elements to form the final shape were obtained from an extensive trial-and-error finite element analysis done to minimise stress levels in a previous blade with a similar profile.
A classical annual energy production optimised chord distribution is shown in FIG. 2. The scale of the chord direction in FIG. 2 is enlarged as compared to the scale of the radial direction. The chord distribution is characterised by a sharp peak of the chord (6) at the shoulder (3) and a concave chord distribution from the shoulder (3) to the tip point (4). The part between the blade root (2) and the shoulder (3) does not contribute notably to the annual energy production, and it is merely a structural carrier of loads between the annual energy producing part of the blade and the blade root.
For practical reasons an annual energy production optimised chord distribution is normally not applied directly, since the shoulder chord becomes too large for convenient manufacturing, transportation and handling. Consequently, chord distributions in common use today tend to be modifications of annual energy production optimised layout, as depicted in FIG. 3. The dashed outer blade chord distribution (10) is a simple downscaling of an annual energy production optimised chord distribution, with moderately reduced output. The full line (11) and dash-dot line (12) outer blade chord distribution have been modified by e.g. manufacturing or structural considerations.